Remove invalid releases automatically (#466)

Automatically remove empty releases or releases which are released in the future, regardless of the product.

Also refactored a bit releasedata.py to use better names and clarify types.

releases/postmarketos.json

@@ -1,33 +1,5 @@
 {
-  "releases": {
-    "25.06": {
-      "name": "25.06"
-    },
-    "24.12": {
-      "name": "24.12"
-    },
-    "24.06": {
-      "name": "24.06"
-    },
-    "23.12": {
-      "name": "23.12"
-    },
-    "23.06": {
-      "name": "23.06"
-    },
-    "22.12": {
-      "name": "22.12"
-    },
-    "22.06": {
-      "name": "22.06"
-    },
-    "21.12": {
-      "name": "21.12"
-    },
-    "21.06": {
-      "name": "21.06"
-    }
-  },
+  "releases": {},
   "versions": {
     "25.06": {
       "name": "25.06",